It's that time of the season when healthcare companies begin peppering investors with a flurry of earnings reports. If you still aren't sure which ones to watch, don't sweat it.

At least four healthcare companies still scheduled to report this week deserve your attention. Read on to find out why.

Company (Symbol) Scheduled Date  1-Year Stock Performance
Amarin (NASDAQ:AMRN) Wednesday, May 1 577%
Exact Sciences (NASDAQ:EXAS) Tuesday, April 30 89%
Merck & Co. (NYSE:MRK) Tuesday, April 30 29%
Amgen (NASDAQ:AMGN) Tuesday, April 30 2%

Data source: Company releases and Yahoo! Finance.

1. Amarin: Can Vascepa sales keep accelerating?

Last year, Amarin blew the lid off its stock price with better-than-expected results from the Reduce-It trial with its special fish oil pill, Vascepa. This study enrolled people already using statins to lower their cholesterol if they also have dangerously high triglycerides, a description that fits an estimated 9.5 million Americans.

Adding a daily dose of Vascepa to regular statin therapy reduced the risk of a major adverse cardiovascular event (MACE) by 25% compared with a placebo, and sales have been accelerating ever since. Vascepa revenue rose 44% in the fourth quarter to an annualized $309 million. 

When Amarin reports first-quarter results on Wednesday morning, investors will be looking for a continuation of unexpected early revenue growth in response to the results. Three months ago, the company expected Vascepa sales to rise by at least 50% to around $350 million in 2019, and an upward revision could be in the cards. 

Amarin submitted its application to add a new indication to Vascepa's prescribing label about a month ago, and the Food and Drug Administration still has another month to officially begin a review that will probably last until January. An eventual green light that allows Amarin to officially market Vascepa as a treatment to lower MACE risk is expected to drive revenue from $229 million in 2018 to $1.9 billion by 2024.

Happy guy in a lab coat using a pipette.

Image source: Getty Images.

2. Exact Sciences: Reaching the other 95.9%

When Exact Sciences reports first-quarter earnings on Tuesday afternoon, investors will be looking for signs that Cologuard can continue growing its share of eligible patients from 4.1% at the end of 2018 to a double-digit percentage soon. The company thinks there are around 85 million Americans between 50 and 85 who should get screened every three years. Thanks to the Affordable Care Act, insurers can't charge their customers for recommended Cologuard screenings.

The company completed 934,000 tests last year, which was 66% more than a year earlier, thanks to bombarding Americans with advertisements. Last August, the company hired Pfizer (NYSE:PFE) to help promote Cologuard, and sales appear to have accelerated in the fourth quarter of 2018. If it still looks like this marketing collaboration will help Exact Sciences reach a much larger share of eligible patients, the stock could soar.

3. Merck: Can Keytruda become the world's top-selling drug?

In 2018, Merck's lead drug, Keytruda, earned approval to treat six new indications, including patients newly diagnosed with the most common forms of lung cancer. Investors will be looking for signs Keytruda can become the world's top-selling drug in 2025, when Merck reports first-quarter earnings on Tuesday morning.

Keytruda sales rose 66% in the fourth quarter to an annualized $8 billion. With a leading share of the important lung cancer population, there's a pretty good chance Keytruda can continue climbing at its recent pace for at least another year or two.

Scientist in the laboratory looking at charts and graphs.

Image source: Getty Images.

4. Amgen: Still in the lead?

Last year, Amgen was the first of three companies to launch a new type of migraine prevention treatment, Aimovig. There are millions of Americans who could benefit from fewer migraine days each month, making Aimovig a huge potential growth driver. Investors will be even more interested in Aimovig now that there's a chance Amgen can wiggle out of a collaboration deal with Novartis.

By the end of 2018, over 150,000 patients tried Aimovig, which worked out to a majority of new migraine drug prescriptions filled last year. When Amgen reports first-quarter earnings on Tuesday afternoon, investors will be looking for signs that Aimovig has maintained its leading share of the migraine space.

Most combustible?

Aimovig could become an important growth driver for Amgen, but the big biotech could probably continue pushing the needle forward if it fizzles out. The rest of these healthcare stocks, though, are extremely dependent on their lead products. 

It was unintentional, but I've listed the stocks in order of combustibility, with Amarin as most likely to go screaming higher, or lower. Keep that in mind if you're going to dig further to see if any of these stocks are worthy of your portfolio.